1. Field
The present application relates generally to the operation and design of analog front ends, and more particularly, to the operation and design of wide band beamforming receivers.
2. Background
In wideband direct conversion receivers employing radio frequency (RF) beamforming, local oscillator (LO) leakage paths through the receiver can give rise to beam dependent DC offset. For example, LO leakage may affect the operation of a phase shifter, power combiner, or the downconverter in the receive chain. As a result, the down-converted signal may include DC offset attributable to this leakage. Furthermore, the DC offset may be beam dependant and therefore different offsets may result based on the selected beam direction or beam pattern.
In some conventional systems, digital filters are used to mitigate DC offset. For example, an analog-to-digital converter (ADC) is used to convert the analog down-converted signal to a digital signal for further processing. To mitigate the DC offset, digital filters are inserted after the ADC. However, such filters do not help if the DC offset saturates the ADC. Furthermore, the digital filters typically provide a wideband notch that can impact single-carrier performance.
In other conventional systems, a beam dependent calibration is employed to cancel the DC offset. In these systems, digital-to-analog converters (DAC) are programmed to provide correction values that are used in the receive signal path to cancel the DC offset. However, this solution is inefficient in that it is time-consuming to calibrate all the correction values for a large number of beam directions or beam patterns. Furthermore this solution is complex to implement in that additional circuitry may be needed to store and switch different DC correction values into the receive chain whenever the beam direction/pattern is changed.
Accordingly, it would be desirable to have a simple and efficient mechanism to remove or reduce DC offset in wideband direct conversion receivers employing RF beamforming.